This is in Libreoffice version 3.4.5, latest in Sid. With Libreoffice-kde installed, the tooltips are rendering in black on black or black on very dark gray, making them essentially impossible to read. Besides uninstalling libreoffice-kde, the other workaround I've found is setting the system Widget style in System Settings / Application Appearance to GTK+ Style. All other widget styles give the unreadable tooltips. I have not found any workarounds in LibreOffice settings. Not the first widget issues with libreoffice-kde, of course. I like libreoffice-kde mainly for the consistency of the dialog boxes.

I have got the same problem and I want to keep the oxygen color sheme.
I only wonder why the text-color of the oxygen color sheme (white) isn't displayed by GTK-based applications.
Instead, black is used as the text color, which results in black text on a dark background, as alexk already described it.
Is it possible to change this behaviour of GTK applications?

Have you tried KDE's System Settings/Application Appearance/Colours/ Colours tab and setting the tooltips background to some light colour ( I think light blue is the default)?

This seems to work, thanks. However, LibreOffice doesn't respond to any Tooltip text color selections, only to background selections. Other GTK apps such as Gscan2pdf respond to both the background and text color selections. I'm using a customized desktop theme and chose a less dark tooltip background.

The procedure above is just a workaround as this issue didn't exist in previous recent versions of LibreOffice. Looks like a bug in libreoffice-kde to me. No way to control the font color of tooltips, it's always black.

Edit: Oops, problem not solved. Most button tooltips now respond to tooltip font color selections, but some still display in black only, regardless of the color selection. Additionally, many of my menu headers and items don't display anymore, instead there are dashes where the text should be.